Late Jurassic Extension Research Articles

Decoding the Link Between Magmatic Cyclicity and Episodic Variation of Tectonics and Crustal Thickness in the Overriding Plate

AbstractCyclical change in subduction angle is the favorable mechanism to elucidate the cyclicity of continental arc magmatism, however, the role of episodic tectonics and variation of the lithosphere in overriding plates is much underestimated. Here we focus on structural, magnetic, and gravitational features of the Late Jurassic to Early Cretaceous granites in the Mesozoic Paleo‐Pacific arc system of the North China block. By unraveling the emplacement process and regional tectonics, we establish a three‐staged extension‐contraction cycle with crustal thickness variation controlling the magmatic flux and behavior. The Late Jurassic extension produced high‐flux crustal‐derived magma (1.87 × 103 km2/Myr), but the thick crust >45 km accumulated large granitic batholiths by multi‐feeders emplacement at the middle‐lower crust and prevented magma ascent and eruption. Subsequently, the Latest Jurassic to Earliest Cretaceous contraction resulted in the magmatic lull and thickened crust of ca. 60 km, fueling crustal material for the ensuing magmatism. In the Early Cretaceous, intense crustal extension thinned the crust to 30 km and largely enhanced the magmatic flux (3.03 × 103 km2/Myr). The magma is prone to penetrate the thin crust with an intensive eruption. A small amount of magma was stored, and the emplacement was controlled by ductile detachments or normal faults. Our model emphasizes episodic the deformation of lithosphere and associated crustal thickness variation in controlling magma production, which may shed new light in understanding the magmatic cyclicity under continuous subduction.

Late Jurassic back‐arc extension in the Neuquén Basin (37°S): Insights from structural, sedimentological and provenance analyses

AbstractThe Middle Jurassic–Early Cretaceous evolution of the Neuquén Basin is traditionally attributed to a long phase of thermal subsidence. However, recent works have challenged this model. In view of this, we study the Late Jurassic Tordillo Formation, a non‐marine depositional unit that marks a shift to regional regression across the basin. Previous studies propose different causes for this regression, including the growth of the magmatic arc in the west, uplift in the south or extension in the north. We studied the Tordillo Formation in sections located at an intermediate position in the Neuquén Basin, in order to understand the tectonic processes active during sedimentation. We present evidence of normal faulting within the Tordillo Formation and the base of the overlying Vaca Muerta Formation. Some of these faults can be attributed as syndepositional. We characterize the Tordillo Formation as part of a distal fan‐playa lake depositional system with a contemporaneous western magmatic arc as the main source of sediment. When compared to the Late Triassic–Early Jurassic NE to NNE‐oriented rifting, which marks the opening of the Neuquén Basin, the Late Jurassic extension shows a switch in stress orientation; the latter is orthogonal to the north‐trending subduction zone. We interpret this change as a renewed phase of back‐arc extension induced by slab rollback along with minor distributed intraplate extension prior to opening of the South Atlantic Ocean.

Tectono‐stratigraphic development of a salt‐influenced rift margin: Halten Terrace, offshore Mid‐Norway

AbstractPre‐rift salt controls structural style variability within rifts by decoupling sub‐ and supra‐salt faults. However, the way in which this variability controls sediment erosion and dispersal, and facies distributions within the coeval syn‐rift stratigraphic succession, remains poorly known. We here use 3D seismic reflection and borehole data to study the tectono‐stratigraphic development of the Halten Terrace, offshore Mid‐Norway, a salt‐influenced rifted margin formed during Middle to Late Jurassic extension. On the eastern basin margin, the rift structural style passes southwards from an unbreached extensional growth fold dissected by numerous horst and graben (Bremstein Fault Complex [BFC]), into a single, through‐going normal fault (Vingleia Fault Complex [VFC]). This southwards change in structural style is likely related to the pinch‐out of or a change in the dominant lithology (and thus rheology) within a pre‐rift (Triassic) evaporite layer, which was thick and/or mobile enough in the north to decouple basement‐ and cover‐involved faulting, and to permit extensional forced folding. As a result, the salt‐influenced BFC underwent limited footwall uplift, with minor erosion of relatively small horsts supplying only limited volumes of sediment to the main downdip depocentre. In contrast, the VFC, which was directly coupled to basement, experienced significant uplift and extensive footwall erosion. The footwall of this structure also locally underwent salt‐detached gravity gliding and collapse as the pre‐rift detachment was tilted. Our results show that where through‐going normal faults develop along the rift flanks, the presence of a pre‐rift salt layer will suppress the topographic expression of the footwall. The pre‐rift salt layer may however facilitate footwall collapse and limit the volume of sediment supplied to downdip basins. Our results also show that variable topography along the rift flanks facilitated the development of relatively small, localised, intra‐rift flank accommodation that trapped flank‐derived sediment, and which meant basins nearer the rift axis were starved of sediment.

Influence of attenuated lithosphere and sediment loading on flexure of the deep-water Magallanes retroarc foreland basin, Southern Andes

Flexural subsidence in foreland basins is controlled by applied loads—such as topography, water/sediment, and subcrustal forces—and the mechanical properties of the lithosphere. We investigate the controls on subsidence observed within the Upper Cretaceous Magallanes retroarc foreland basin of southern South America to evaluate the impact of lateral variations in flexural rigidity due to Late Jurassic extension. Conventional elastic models cannot explain the observed basin deflection and thick accumulation of deep-water Cenomanian-Turonian basin strata. However, models in which the lithosphere has been previously thinned and deflects under topographic and sedimentary loads successfully reproduce regional subsidence patterns. Results satisfy paleobathymetric observations in the Magallanes Basin and suggest that lithospheric thinning is necessary to produce both long-wavelength and deep subsidence during Late Cretaceous basin evolution. Results indicate that elastic thickness decreases westward from ~45–25 km in the distal foreland to ~37–15 km beneath the foredeep. These findings are consistent with a westward reduction in crustal thickness associated with the Jurassic extensional history of the Patagonian lithosphere. Our results also show that sediment loading exerts an important control on regional deflection patterns and promotes a wider region of subsidence and reduced forebulge uplift. We propose that lateral variations in mechanical properties and large sediment loads restrict depocenter migration and may cause the foredeep to remain fixed for prolonged periods of time. These findings confirm that loading of thinned lithosphere imposes different mechanical controls on the flexural profile and have potential implications for other retroarc foreland basins characterized by earlier extensional histories.

Formation of the giant Chalukou porphyry Mo deposit in northern Great Xing'an Range, NE China: Partial melting of the juvenile lower crust in intra-plate extensional environment

Abstract The Chalukou porphyry Mo deposit (2.46 Mt @ 0.087% Mo), located in the northern Great Xing'an Range, NE China, is the largest Mo deposit discovered in China so far. The host rocks consist of aplite porphyry, granite porphyry and quartz porphyry, and are intruded into Lower Ordovician intermediate-felsic volcanic–sedimentary rocks and pre-ore monzogranite and are cut by post-ore feldspar porphyry, diorite porphyry and quartz monzonite porphyry. Here, we present the zircon U–Pb ages, whole-rock geochemistry, Sr–Nd isotopic and zircon Hf isotopic data for the pre-ore, syn-ore and post-ore intrusive rocks. The Chalukou ore-forming porphyries intruded during 147–148 Ma and have high-silica, alkali-rich, metaluminous to slightly peraluminous compositions and are oxidized. They are enriched in large ion lithophile elements (e.g. K, Rb, U and Th), light REE and depleted in high-field strength elements (e.g. Nb, P and Ti). Depletions in Eu, Ba, Sr, Nb, Ta, P and Ti suggest that they have experienced strong fractional crystallization of plagioclase, biotite, hornblende and accessory minerals. The pre-ore monzogranite (~ 172 Ma) also belongs to the high-K calc-alkaline series. Highly fractionated REE patterns ((La/Yb) N = 19.6–21.7), high values of Sr/Y (54–69) and La/Yb (29–32), are adakite-like geochemical features. The post-ore rocks (~ 141–128 Ma) have similar geochemical characteristics with ore-forming porphyries except that quartz monzonite porphyry shows no Ba–Sr negative anomaly. All intrusive rocks have relative low initial 87Sr/86Sr (0.705413–0.707889) and eNd (t) values (− 1.28 to + 0.92), positive eHf (t) values (+ 2.4 to + 10.1) and young two-stage Nd and Hf model ages (TDM2 (Nd) = 863–977 Ma, TDM2 (Hf) = 552–976 Ma). These geochemical and isotopic data are interpreted to demonstrate that the ore-forming porphyries formed by partial melting of the juvenile lower crust caused by underplating of mafic magmas in an intra-plate extensional setting. The pre-ore monzogranite formed by partial melting of thickened lower crust in a collisional setting caused by closure of Mongol–Okhotsk Ocean. The post-ore feldspar porphyry shares a similar magma source with ore-forming porphyry, but the quartz monzonite porphyry has a relatively deeper magma source region and has not experienced as much fractional crystallization. The transformation from middle Jurassic compression to late Jurassic extension created favorable conditions for the generation and emplacement of the ore-forming magma. The juvenile lower crust provided the main source of molybdenum for Chalukou deposit. Enrichment of Mo by fractional crystallization played an important role in concentrating Mo during formation of the Chalukou Mo deposit. The age (~ 147 Ma), high fluorine, and associated Pb–Zn deposits are all different from other major porphyry Mo deposits in NE China; Chalukou is a new mineral deposit type in the Great Xing'an Range.

Normal fault array evolution above a reactivated rift fabric; a subsurface example from the northern Horda Platform, Norwegian North Sea

AbstractThe impact of a pre‐existing rift fabric on normal fault array evolution during a subsequent phase of lithospheric extension is investigated using 2‐D and 3‐D seismic reflection, and borehole data from the northern Horda Platform, Norwegian North Sea. Two fault populations are developed: (i) a population comprising relatively tall (>2 km), N‐S‐striking faults, which have >1.5 km of throw. These faults are up to 60 km long, penetrate down into crystalline basement and bound the eastern margins of 6–15 km wide half‐graben, which contain >3 km of pre‐Jurassic, likely Permo–Triassic, but possibly Devonian syn‐rift strata; and (ii) a population comprising vertically restricted (<1 km), NW‐SE‐striking faults, which are more closely spaced (0.5–5 km), have lower displacements (30–100 m) and not as long (2–10 km) as those in the N–S‐striking population. The NW‐SE‐striking population typically occurs between the N‐S‐striking population, and may terminate against or cross‐cut the larger structures. NW–SE‐striking faults do not bound pre‐Jurassic half‐graben and are largely restricted to the Jurassic‐to‐Cretaceous succession. Seismic‐stratigraphic observations, and the stratigraphic position of the fault tips in both fault populations, allow us to reconstruct the Late Jurassic‐to‐Early Cretaceous growth history of the northern Horda Platform fault array. We suggest the large, N‐S‐striking population was active during the Permo–Triassic and possibly earlier (Devonian?), before becoming inactive and buried during the Early and Middle Jurassic. After a period of relative tectonic quiescence, the N‐S‐striking, pre‐Jurassic fault population propagated through the Early‐Middle Jurassic cover and individual fault systems rapidly (within <10 Ma) established their maximum length in response to Late Jurassic extension. These fault systems became the dominant structures in the newly formed fault array and defined the locations of the main, Late Jurassic‐to‐Early Cretaceous, syn‐rift depocentres. Late Jurassic extension was also accommodated by broadly synchronous growth of the NW‐SE‐striking fault population; the eventual death of this population occurred in response to the localization of strain onto the N–S‐striking fault population. Our study demonstrates that the inheritance of a pre‐existing rift fabric can influence the geometry and growth of individual fault systems and the fault array as a whole. On the basis of observations made in this study, we present a conceptual model that highlights the influence of a pre‐existing rift fabric on fault array evolution in polyphase rifts.

Source-rock maturation characteristics of symmetric and asymmetric grabens inferred from integrated analogue and numerical modeling: The southern Viking Graben (North Sea)

We present the results of an integrated analogue and numerical modeling study with a focus on structural, stratigraphic and thermal differences between symmetric and asymmetric grabens. These models enable fault interpretation and subsidence analyses in studies of active rifting and graben migration. We imported the surface topography and crustal thinning factors from the analogue models into tectono-stratigraphic forward models which allowed an assessment of the relative importance of sediment stacking in the generation of symmetric and asymmetric grabens. Effects on source-rock maturation zones were calculated through 1D-thermal modeling for different graben types. Combined analogue–numerical modeling appears to be a useful method to simulate natural examples, as shown in this study of the southern Viking Graben in the northern North Sea. This area was formed by Early Permian–Late Jurassic extension, with rifting most intense during the Late Jurassic. The thermal structure of the model, constrained by lithospheric and sedimentary parameters for this region, compares well with actual source-rock maturation data on the southern Viking Graben.

Fluid systems and basin evolution of the western Lower Saxony Basin, Germany

AbstractThe western Lower Saxony Basin is characterized by magnetic and gravity anomalies and an unusually strong coalification of surface‐near sedimentary rocks, indicating that high temperatures were reached in the past. The temperature history of the basin has been disputed for many years leading basically to two different hypotheses: one proposes igneous intrusion during Mid Cretaceous time, causing the high maturity in this area, whereas the other theory explained the coalification as a result of deep burial during late Jurassic and early Cretaceous times. Petrographical data are summarized which show the maturity distribution within the basin, reflecting maximum palaeotemperature conditions. This maturity distribution is visualized along three cross sections of up to 50 km length, as well as the palaeotemperature distribution for six time steps. Furthermore, geochemical data on petroleum source rocks are presented which indicate early mature/immature type I/II kerogen in Wealden and Posidonia Shales and mature/overmature type III kerogen in the coal‐bearing Upper Carboniferous. Fluid inclusion compositions in quartz crystals are highly variable ranging from liquid to gaseous, and gas compositions are also variable. These data were interpreted in the context of burial history. This study provides new data and numerical models with the intention to clarify the cause of the high coalification in the basin. 2D structural and basin modelling along three cross sections, combined with new fluid inclusion measurements and results of previous coalification studies, 1D modelling, and fission track dating demonstrate high heat flow during the Late Jurassic extension stage followed by a deep burial of the basin during the Early Cretaceous, associated with only moderate heat flows. The presence of an igneous intrusion of Cretaceous age is not supported. Furthermore, timing of hydrocarbon generation from the three major source rocks is discussed.

Spatio-temporal evolution of strain accumulation derived from multi-scale observations of Late Jurassic rifting in the northern North Sea: A critical test of models for lithospheric extension

We integrate observations of lithospheric extension over a wide range of spatial and temporal scales within the northern North Sea basin and critically review the extent to which existing theories of lithospheric deformation can account for these observations. Data obtained through a prolonged period of hydrocarbon exploration and production has yielded a dense and diverse data set over the entire Viking Graben and its flanking platform areas. These data show how syn-rift accommodation within the basin varied in space and time with sub-kilometer-scale spatial resolution and a temporal resolution of 2–3 Myr. Regional interpretations of 2D seismic reflection, refraction and gravity data for this area have also been published and provide an image of total basin wide stretching for the entire crust. These image data are combined with published strain rate inversion results obtained from tectonic subsidence patterns to constrain the spatio-temporal evolution of strain accumulation throughout the lithosphere during the 40 Myr (170–130 Ma) period of Late Jurassic extension across this basin. For the first 25–30 Myr, strain localisation dominated basin development with strain rates at the eventual rift axis increasing while strain rates over the flanking areas declined. As strain rates across the whole basin were consistently very low (< 3 × 10 - 16 s - l ), thermally induced strength loss cannot explain this phenomenon. The strain localisation is manifest in the near-surface by a systematic migration of fault activity. The pattern and timing of this migration are inconsistent with flexural bending stresses exerting an underlying control, especially when estimates of flexural rigidity for this area are considered. The best explanation for what is observed in this time period is a coupling between near-surface strain localisation, driven by brittle (or plastic) failure, and the evolving thermal structure of the lithosphere. We demonstrate this process using a continuum mechanics model for normal fault growth that incorporates the strain rate-dependence of frictional strength observed in laboratory studies. During the final 10 Myr of basin formation, strain accumulation was focused within the axis and strain rates declined rapidly. Replacement of weak crust by stronger mantle material plus crustal buoyancy forces can adequately explain this decline.

Quantitative dynamic modelling of basin development in the central and eastern North Sea region – coaxial stretching and strain localization

A new two-dimensional dynamic lithosphere model is used to simulate the Late Palaeozoic to end Danian evolution of the Norwegian-Danish Basin and the post Permian evolution of the Central North Sea including the Central Graben. The transient heat equation and the equations of motion are solved using the finite element method. The lithosphere deforms by brittle and ductile processes through an elasto-visco-plastic rheology depending on temperature, pressure, strain-rate and material parameters. Strain softening dependent on accumulated strain is incorporated. Deposition, erosion and compaction of sediments are simulated. Results show that it is possible to satisfy observations of crustal structure, sediment thickness and surface heat flow for both basins taking all major tectonic and thermal events into consideration. The evolution of the Norwegian-Danish Basin is modelled using a Late Carboniferous – Early Permian thermal event, main rift phase in Early Permian and a minor extensional phase in Triassic. For the Central North Sea two thermal and three tectonic events are simulated: Late Carboniferous – Early Permian and Middle Jurassic thermal events, Early Triassic and Late Jurassic extension, and Late Cretaceous compression. Results show that strain softening may lead to strain localization during extension and therefore may explain observations of upper mantle dipping reflectors in the North Sea. A pure shear dominated extensional regime may change into a simple shear system.

Post-Permian evolution of the Central North Sea: a numerical model

A dynamic thermomechanical model is presented for the post-Permian evolution of the Central North Sea including the Central Graben. Permo-Carboniferous and Jurassic thermal events, as well as Triassic and Jurassic extensional phases and Late Cretaceous shortening, are considered. The equations of force balance for a continuum and the transient heat equation are solved using the finite element method. The lithosphere responds to applied forces by elastic deformation at low deviatoric stress levels, fracture at low temperature and pressure conditions and nonlinear viscous flow at high temperature and pressure. A first-order rheological strain softening is incorporated to simulate a transition from strong to soft material behaviour, such as a change from dislocation creep to diffusion creep. Seismic refraction and reflection data indicate crustal thinning below the Central Graben by a factor of ∼2. While the Triassic rifting appears to have been associated with a more distributed deformation pattern, the Upper Jurassic rifting is more localised. Pure shear extensional models have been advocated to explain this. However, dipping seismic reflectors in the upper mantle indicate that simple shear could be important. The strain softening of the present model allows for modelling of strain localisation during the Upper Jurassic rifting phase and detailed modelling of Moho structures. The present model accounts for the main structural and stratigraphic observations in the Central North Sea. Furthermore, the results indicate that in the part of the Central North Sea investigated in this paper, a pure shear regime in Triassic changed into a simple shear-dominated regime with mantle shear zones developing during Late Jurassic extension.

Detachments and low-angle faults in the northern North Sea rift system

Abstract Several master faults in the North Sea basin tend to flatten to give low dips at depth, and in this sense form detachments in the rift system. Such low angle faults are identified in the western flank of the Viking Graben (Tampen Spur area), where they occur as both intra- and supra-basement detachments. Interference between detachments and steeper faults results in ramp-flat-ramp geometries. In the eastern part of the Gullfaks fault block, a supra-basement detachment is probably associated with anomalously high late Jurassic extension in the Gullfaks Field area. The low-angle Gullfaks detachment also helps explain the presence of sets of parallel east-dipping faults (domino systems), a common feature in the collapsed hanging wall to low-angle detachments. Similar detachments probably exist beneath the Gullfaks Sør block and SE of the Visund fault block. All of these are interpreted as late Jurassic collapse structures directly related to active late Jurassic extensional tectonics. Strong indications of intra-basement detachments are also found in the Tampen Spur area. These detachments are formed by major normal faults that flatten in the basement, as seen beneath the Visund fault block. This geometry may to some extent be related to fault rotation during repeated phases of extension in the Palaeozoic-Early Mesozoic period. However, abrupt flattening of some of the faults in the basement indicates that the master faults follow some of the many pre-existing mechanically weak zones in the basement, primarily low-angle Devonian extensional shear zones or Caledonian thrusts.

Late Jurassic subsidence and passive margin evolution in the Vulcan Sub‐basin, north‐west Australia: constraints from basin modelling

The Vulcan Sub‐basin, located in the Timor Sea, north‐west Australia, developed during the Late Jurassic extension which ultimately led to Gondwanan plate breakup and the development of the present‐day passive continental margin. This paper describes the evolution of upper crustal extension and the development of Late Jurassic depocentres in this subbasin, via the use of forward modelling techniques. The results suggest that a lateral variation in structural style exists. The south of the basin is characterized by relatively large, discrete normal faults which have generated deep sub‐basins, whereas more distributed, small‐scale faulting further north reflects a collapse of the early basin margin, with the development of a broader, ‘sagged’ basin geometry. By combining forward and reverse modelling techniques, the degree of associated lithosphere stretching can be quantified. Upper crustal faulting, which represents up to 10% extension, is not balanced by extension in the deeper, ductile lithosphere; the magnitude of this deeper extension is evidenced by the amount of post‐Valanginian thermal subsidence. Reverse modelling shows that the lithosphere stretchingfactor has a magnitude of up to β=1.55 in the southern Vulcan Sub‐basin, decreasing to β=1.2 in the northern Vulcan Sub‐basin. It is proposed that during plate breakup, deformation in the Vulcan Sub‐basin consisted of depth‐dependent lithosphere extension. This additional component of lower crustal and lithosphere stretching is considered to reflect long‐wavelength partitioning of strain associated with continental breakup, which may have extended 300–500 km landward of the continent–ocean boundary.

Sedimentary Basins of the Republic of Yemen : Their Structural Evolution and Geological Characteristics

The distribution and evolution of the sedimentary basins of Yemen was, until recently, poorly understood as this was based entirely on surface geology and correlations of the older stratigraphic units which were exposed only in the deeply dissected bordering uplifts of the Gulf of Aden and Red Sea or the high plateau of the north west. Elsewhere cover by the tabular Tertiary sedimentary blanket and the Tertiary Volcanic Group lavas masked the major underlying pre-Cenozoic structural elements and sedimentary successions. Earlier attempts at the delineation of the country's structural framework were, thus, sketchy and/or only partially correct. The discovery of commercial oil and gas in several interior Mesozoic rift basins of Yemen in the late 1980s and in the early 1990s after unification of the former two Yemens, spured many oil companies to enter the exploration race and carry out detailed seismic surveys and intensive exploration drilling in many areas. This resulted in a rapid rise in overall new subsurface geological data acquisition and an increasingly clearer perception of the distribution, orientation and inception times of the main basins. No overall synthesis of results was, however, undertaken since each individual company was primarily concerned with its own concession area and its immediate surroundings. Recent studies involving the review, correlation and synthesis of the mass of new subsurface stratigraphic data in connection with standardisation of lithostratigraphic nomenclature in use in Yemen and its further formalisation in accordance with internationally accepted rules, have, perforce, required the establishment of an overall structural framework within which inter and intra-basinal stratigraphic correlation could be carried out. It is this new framework of depositional basins and interbasinal uplifts that is discussed here. The main Mesozoic basins are related to late Jurassic extension and rifting, principally involving rejuvenation of the ancient NW-SE Najd fault system with subordinate strike-slip motion. The Tertiary basins are related to separation of Arabia from Africa in the Neogene along the Gulf of Aden and Red Sea trends. The principal uplifts are of equal antiquity, the oldest and most persistent being the Hadhramawt Arch. All the tectonic activity is linked to Gondwanan fragmentation and breakup from the initial closure of the paleo-Tethys and opening of the neo-Tethys in late Carboniferous to Permian times, and the development of the African Karoo system, through separation of India from Afro-Arabia in the Cretaceous to ultimate separation of Arabia from Africa in the Neogene. Within individual basins, compartmentalisation by sub-parrallel highs into graben-like and half-graben sub-basins or sectors is related to local adjustments to this continuing motion within the overall structural framework.

Geodynamic evolution of the Antarctic Peninsula during Mesozoic times and its bearing on Weddell Sea history

This review of the tectonic evolution of the Antarctic Peninsula during Mesozoic times highlights four main events; (1) Late Triassic-Late Jurassic extension, (2) Late Jurassic-Early Cretaceous dextral transpression, (3) Early Cretaceous extension and (4) mid-Cretaceous compression. Magmatism was virtually continuous during much of this period with the exception of possible breaks in the known record in Early Jurassic and Late Jurassic-Early Cretaceous times. The second of these breaks corresponded to the first compressional event. There was no apparent hiatus in the magmatic record during the mid-Cretaceous compressional event, although there was a significant change in the pattern of sedimentation in the Larsen basin on the eastern margin of the Weddell Sea at about that time. The tectonic evolution of the peninsula is compared to, and puts some constraints on, existing Weddell Sea models. The Late Triassic-Late Jurassic arc extension correlates with initial rifting in the Weddell Sea region during sinistral motion between East and West Gondwana. However, there is no known record of large-scale pre-Mid-Jurassic transcurrent deformation in the Antarctic Peninsula that would have been consistent with rotation of West Antarctic crustal blocks in the initial rifting period. The peninsula-wide Late Jurassic-Early Cretaceous compressional event may correlate with the major change in Gondwana plate motions from E-W to N-S (African reference frame) separation of East and West Gondwana. This change probably resulted in formation of an ocean-continent boundary along the northern margin of the Weddell Sea embayment and initial seafloor spreading. Geological data do not seem to support subduction of southwestern proto-Weddell Sea oceanic lithosphere beneath the eastern margin of the peninsula at that time. Cretaceous arc extension was coeval with the initial seafloor spreading phase in the Weddell Sea. Mid-Cretaceous arc compression, linked to a global increase in ocean floor spreading rates and a superplume event, correlates with a change from NE-SW to NW-SE spreading in the Weddell Sea.

Palaeocene uplift and Eocene subsidence in the northern North Sea Basin from 2D forward and reverse stratigraphic modelling

Structural and stratigraphic basin modelling has been used to examine quantitatively the Cretaceous and Tertiary post-rift subsidence history of the northern North Sea Basin following Late Jurassic extension. 2D forward and reverse, syn-rift and post-rift modelling has been used to determine the magnitude and timing of departures from the McKenzie post-rift thermal subsidence trend for regional stratigraphic profiles in the Outer Moray Firth, the South Viking Graben and the North Viking Graben. Observed stratigraphy has been reverse post-rift modelled using flexural backstripping combined with decompaction and reverse thermal subsidence calculations, and forward modelled through the syn-rift and post-rift period using the flexural cantilever model of continental rift basin formation. Long-term global eustasy has been included in the analysis. Only reliable palaeobathymetric markers, such as erosion surfaces and coals, have been used to constrain subsidence history. Both forward and reverse 2D modelling show that a Palaeocene regional uplift followed by a rapid Eocene subsidence must be superimposed on McKenzie post-rift Cretaceous and Tertiary basement subsidence in order to generate the stratigraphy observed. The magnitude of Palaeocene uplift deduced from modelling was of the order of 375 m in the Outer Moray Firth (central North Sea) increasing to 525 m in the North Viking Graben (northern North Sea). Rapid Early Eocene subsidence was of the order of 160–310 m. The most likely mechanism for regional Palaeocene uplift and rapid Eocene subsidence in the northern North Sea was the Palaeocene development of the Iceland plume situated at a distance of 700–900 km from the study area in the Early Tertiary. Forward and reverse modelling predict axial bathymetries in the graben of up to 1300 m for the Early Cretaceous and 900 m for the Upper Cretaceous. The large palaeobathymetries at the end of the Cretaceous provided much of the accommodation space for Tertiary sedimentation.

Tectonic controls on deposition of Middle Jurassic strata in a retroarc foreland basin, Utah‐Idaho trough, western interior, United States

An electronic supplement of this material may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN to AGU’s FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 95TC01448, Tectonic controls on deposition of Middle Jurassic strata in a retroarc foreland basin, Utah‐Idaho trough, western interior, United States, Christian J. Bjerrum and Rebecca J. Dorsey). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N. W., Washington, DC 20009; $15.00. Payment must accompany order.A thick succession of Jurassic nonmarine and marine sedimentary rocks is exposed in a large area from northern Arizona to eastern Idaho and western Wyoming. These sediments accumulated in the Utah‐Idaho trough, a deep elongate cratonal basin whose origin has recently been debated. Detailed stratigraphic analysis, subsidence analysis, and first‐order flexural modeling of these deposits (this study) provide new insights into the timing and mechanisms of subsidence in the Utah‐Idaho trough. Lower and Middle Jurassic strata are divided into six unconformity‐bounded sequences. In contrast to the overall uniform thickness of Lower Jurassic sequences (1 and 2), Middle Jurassic strata (sequences 3 through 6) consist of distinctly westward thickening sedimentary packages in which basal shallow marine deposits onlap eastward onto bounding unconformities. Basal strata of sequences 3 through 6 pass upward into widespread progradational continental deposits that are truncated progressively toward the east (cratonward) by the next unconformity. Decompacted total subsidence curves show that the rate of subsidence in most sections increased sharply at the end of sequence 2 time (J‐2 unconformity; ∼170 Ma). This is interpreted to record the onset of Middle Jurassic deposition in the distal part of a retroarc foreland basin. The unconformities and distinctive stratal geometries may have formed in response to forebulge migration caused by episodic thrusting in the Cordilleran orogen to the west. First‐order flexural modeling was carried out to test the hypothesis of flexural subsidence in the Utah‐Idaho trough. Trial‐and‐error comparisons produce a close match between decompacted stratigraphic profiles and model deflection profiles. The best fit is obtained using an infinite elastic plate (D = 1 × 1024 Nm), a moderate load topography, elevated base level, and an overfilled basin. Using recently published tectonic reconstructions for Nevada and Utah, we find close spatial agreement between a large Middle Jurassic fold‐thrust belt and the supracrustal load inferred from model simulations. Our integrated basin analysis thus supports the interpretation of some previous studies that the Middle Jurassic Utah‐Idaho trough was a retroarc foreland basin that formed east of a belt of regional contractile deformation and crustal thickening in western and central Nevada and southeastern California. Late Jurassic extension and normal faulting in northeastern Nevada may have been related to gravitational collapse of overthickened crust in the Cordilleran orogen. This coincides with a period of slowed subsidence in the Utah‐Idaho trough that began at about 157 Ma.

Jurassic magmatism in the central Mojave Desert: Implications for arc paleogeography and preservation of continental volcanic sequences

Research Article| June 01, 1994 Jurassic magmatism in the central Mojave Desert: Implications for arc paleogeography and preservation of continental volcanic sequences ELIZABETH R. SCHERMER; ELIZABETH R. SCHERMER 1Department of Geology, Western Washington University, Bellingham, Washington 98225 Search for other works by this author on: GSW Google Scholar CATHY BUSBY CATHY BUSBY 2Department of Geological Sciences, University of California, Santa Barbara, California, 93106 Search for other works by this author on: GSW Google Scholar Author and Article Information ELIZABETH R. SCHERMER 1Department of Geology, Western Washington University, Bellingham, Washington 98225 CATHY BUSBY 2Department of Geological Sciences, University of California, Santa Barbara, California, 93106 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1994) 106 (6): 767–790. https://doi.org/10.1130/0016-7606(1994)106<0767:JMITCM>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ELIZABETH R. SCHERMER, CATHY BUSBY; Jurassic magmatism in the central Mojave Desert: Implications for arc paleogeography and preservation of continental volcanic sequences. GSA Bulletin 1994;; 106 (6): 767–790. doi: https://doi.org/10.1130/0016-7606(1994)106<0767:JMITCM>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Exposures of Jurassic magmatic rocks in the west-central Mojave Desert provide insight into the changing character of volcanism throughout Jurassic time and the paleogeography of the continental arc. Middle Jurassic explosive volcanism (Lower Sidewinder volcanic series) resulted in collapse of multiple calderas. This was followed by north-south extension broadly coeval with batholith emplacement. Late Jurassic effusive volcanism (Upper Sidewinder volcanic series) appears to reflect transtension over a larger region during intrusion of the Independence dike swarm.The Lower Sidewinder volcanic series consists of a subaerial, nested caldera complex with an aggregate thickness of >4 km. The first caldera formed during the eruption of crystal-poor rhyolite ignimbrite. Outflow and intracaldera facies are intercalated with quartzose sandstone of probable cratonal provenance. The second caldera formed during the eruption of crystal- rich rhyolite to dacite ignimbrite. This ignimbrite exhibits complex mineralogical zoning and contains two units of probable collapse-related mesobreccia within the 1,400-m-thick intracaldera sequence. A third caldera, nested within the second, is filled with crystal-rich biotite-dacite ignimbrite and tuff breccia. A fourth caldera, largely coincident with the second, formed during eruption of lithic- and pumice-lapilli dacite ignimbrite. Eruption and collapse appear to have been multistage, as several units of reworked tuff and fallout tuff occur within the 1,750-m-thick intracaldera sequence, and caldera collapse breccias occur at the base and near the middle of the sequence. The top of this sequence contains reworked tuffs and epiclastic rocks, suggesting that the fourth caldera provided a posteruptive depocenter for accumulation of sediment.Normal faulting, tilting, and erosion followed explosive volcanism and was broadly contemporaneous with intrusion of Middle Jurassic porphyritic quartz monzonite plutons. Plutons and tilted Lower Sidewinder volcanic series are intruded and unconformably over-lain by latest Jurassic volcanic rocks (Upper Sidewinder volcanic series). The Upper Sidewinder volcanic series is characterized by alkalic basalt to basaltic andesite and rhyolite lavas, hypabyssal intrusions, and dikes, including the 148 Ma Independence dike swarm. Regional northeast-southwest extension is suggested by the presence of the northwest-striking dike swarm and by bimodal volcanism. The absence of debris-flow deposits and epiclastic rocks suggests an intra-arc region of low relief.Although local caldera-related subsidence was the principal control on accumulation of Lower Sidewinder volcanic rocks, preservation was likely enhanced by Middle Jurassic extension. Late Jurassic extension was of broader regional extent but lesser magnitude because it did not create basins or cause significant tilting of strata. The geometry, timing,and regional setting of Middle and Late Jurassic extension and magmatism suggest a sinistral oblique subduction regime for the Jurassic arc of the southern U.S. Cordillera. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Relationships between basement faulting, salt withdrawal and Late Jurassic rifting, UK Central North Sea

Seismic data are presented to illustrate the variety of structures found at the margins of the Central Graben and intra-graben horsts. The complexity of these structures is due to the interplay of Late Cimmerian basement extension and older salt withdrawal structures within the Zechstein–Jurassic interval. Basement extension occurred in phases from the Permian to Late Jurassic. Salt withdrawal commenced in the Triassic and continued to the Cretaceous, resulting in a network of salt withdrawal synclines and areas of salt preservation, termed ‘pods’ and ‘interpods’, respectively. During Late Jurassic extension and rifting, the interaction of basement normal faults and the overlying Zechstein–Jurassic section resulted in structures which vary according to the precise location of the basement fault beneath a pod–interpod complex. Variation in the type and complexity of structuring along basin margins results from the highly variable nature and degree of salt withdrawal prior to Late Jurassic rifting. Complex graben margin structures frequently lend themselves to be interpreted as wrench faults, with different implications for structural and stratigraphic interpretations compared to salt withdrawal interpretations. Careful evaluation of structural style at graben margins therefore has important implications for both prospect mapping and regional structural models

Structural controls on the Late Jurassic age shelf system, Ula Trend, Norwegian North Sea

The Ula Trend of the Norwegian North Sea is an important producing area sited on the eastern margin of the Central Trough. Two fields, Ula and Gyda (combined reserves of 635 MMBBL), are on stream, and several other petroleum pools have been discovered. They key to success in the area is to predict the presence of Kimmeridgian to Portlandian age shallow-marine shelf sandstone reservoirs. Well and seismic data have shown the reservoir sequences to have a complex distribution controlled largely by the structural topography at the time of deposition. Structural interpretation of the 3D seismic datasets over the Ula and Gyda field areas and dense 2D data over the Ula Trend as a whole have revealed new insights into the geometry of the Jurassic rift margin. A relationship has been established between the Jurassic faults and the underlying salt wall topography to a deeper Early Permian age extensional system. During Late Jurassic extension, the deeper faults cut up-section and involved the Upper Permian, Triassic and Jurassic cover. Original Early Permian dip-slip faults maintained their sense of motion, while a linked oblique-slip set acted in sinistral transtension. The cover section accommodated, the differing styles with both simple extensional structures and more complex transtensional and contractional geometries, respectively. The structural pattern has been further complicated by episodic hangingwall inversion in Cretaceous and Tertiary times and the inversion geometries provide important petroleum traps (e.g. Ula Field). The Jurassic rift margin fault system and the geometry of its hangingwall governed the location of the Kimmeridgian shelf and shelf-break, as well as the entry and dispersal of sediment on the shelf. The structural interpretation has thus allowed a predictive model for shelf geometry and hence reservoir distribution to be established, and this forms the basis for continued exploration in this complex and increasingly mature play fairway.